This invention relates generally to telephone line protector devices. More particularly, this invention relates to a new and improved telephone line protector device which incorporates protection against both transient voltages and high current fluctuations. The protective device is adapted to be insertable directly onto terminals extending upwardly from a terminal block.
Telephone and communications terminal blocks having a plurality of individual finger-like terminals extending therefrom, such as the well known 66-type connector blocks, connect equipment which requires protection from damaging high voltages and current levels. For many years, protective devices utilizing gas tube or carbon mechanisms have been used to protect telephone circuitry from high voltage and current damage. Such "primary" protective devices are located at the juncture between the outside telephone lines and the lines leading into the building known as outside plant (OSP).
Even with the use of primary voltage protective devices, the leading edge of a voltage spike from a lightning strike, by a momentary contact with a high voltage line or by other causes, will pass through the primary device due to the inherent delay in the gas tube or carbon protector. As the nature of telephone switching equipment shifts from the mechanical relay type to electronic and as sensitive computer equipment is added to the communications network, it is increasingly desirable to protect low voltage telephone and data circuits from high transient voltages at the terminal blocks wherein connections are made between the primary protective devices and equipment inside the building protective devices of this type are known as "secondary" protectors. A secondary device is used with the primary protector as a second line of defense against any surge currents or transient voltage spikes that pass through the primary protection. Also, the secondary device is located as close as possible to the equipment connection point such that any transients generated between building entrance and the main distribution point are clamped as well.
Such "secondary" protective devices are known which include electrical components for protecting against high transient voltages and which plug directly ontO the terminals of a terminal block. Examples of plug-on transient voltage protectors are disclosed in U.S. Pat. Nos. 4,113,340, 4,116,524, 4,126,369, 4,554,609 and 4,654,743.
Discrete devices have also been utilized to protect telephone circuits from high currents (as opposed to high voltages) which manage to pass through the "primary" protection. These secondary protectors are known as "sneak current protectors". An early sneak current protector manufactured by Western Electric Company is known in the communications field as a type 60D fuse. The 60D fuse is a relatively large and bulky fuse which attaches to the fanning strip of a 66 M type connecting block When attached, the 60D fuse both extends laterally from the block and above the top of the block. As a result, the use of this fuse precludes efficient terminal block spacing and the use of block covers. In addition, the type 60D fuse necessitates intricate and time consuming handwiring.
U.S. Pats. Nos. 4,126,369 and 4,447,105 disclose an improved sneak current protector which is installed directly onto adjacent terminals of a 66 type terminal block. However, there are several important disadvantages and drawbacks to this device. For example, these sneak current protectors may comprise a rectangular housing having a relatively large cylindrical attachment (housing a fuse) attached to one side of the housing and extending upwardly from the housing. The presence of the bulky cylinder necessitates alternate mounting of the devices, precludes side to side stacking and precludes the use of a cover. Moreover, the unit is relatively difficult to assemble and install due to its multiplicity of parts, all of which leads to higher manufacturing and installation costs. Also, when the fuse breaks, the entire unit must be discarded and replaced.
An improved sneak current protector specifically adapted for plugging directly to a terminal block is disclosed in U.S. Application Ser. No. 115,531 filed Oct. 30, 1987, assigned to the assignee hereof, all of the contents of which are incorporated herein by reference. This improved sneak current protector comprises a plastic housing which incorporates a pair of small, easily replaceable cylindrical fuse elements.
Notwithstanding the above-mentioned prior art protective devices, there continues to be a need for improved economical plug-on protectors which combine protection from both damaging transient voltages and current overloads.